two-day-a-year-jew

A Jew, usually reformed but he or she can also be conservative if they go to a large enough temple, who only goes to temple for the two days of Rosh Hannah, though occasionally they may show up for Yom Kippur (although the fasting impairs socializing). They usually spend the most of their time socializing outside with their friends.

"I didn't even know Michael went to this temple!"
"Oh you didn't know? He's been a two-day-a-year-jew since his Bar Mitzvah."